The recent world-wide energy shortage has provoked much technical interest in development of energy sources other than those relying on oil and natural gas. Among these alternative sources is geothermal energy which has been viewed, in particular, as a source for electrical power generation and has therefore been recently considered as an economically attractive alternate to fossil fuels. Geothermal energy does not typically result in production of atmospheric pollutants and where capital and operating cost benefits are achievable, substantial advantages over conventional energy generation systems may be realized.
Geothermal energy sources usually are based on either a supply of subsurface steam or subsurface superheated hot water. The latter sources are substantially more common than are the former and consequently much effort has gone into development of methods to convert geothermal hot water into electrical power.
The normal energy recovery procedure includes the steps of withdrawing superheated water from a geothermal well and thereafter flashing it to form a steam phase, separating the available steam at the surface in a flash drum or similar separator and thereafter driving a steam turbine with the steam, and ultimately disposing of the spent hot water.
Among the significant problems faced by those employing geothermal hot water wells for the generation of electrical energy is the deposition of salts in the well and in the separators and power generation equipment. Typically geothermal brines have extremely high levels of dissolved solids and as the brines are cooled and made more concentrated by release of substantial parts of their water content in the flash step, the solids tend to deposit in the wellbore, the valves or pipes, the flash drums and other related equipment. This disadvantageous result is in particular encountered where the geothermal brines are supersaturated in silica.
The silica deposition necessitates either frequent cleaning of the wellbore and associated piping and steam separators, which is difficult and time consuming as well as expensive and disruptive of the entire operation, or, requires use of expensive chemical control methods involving introduction of additives to the geothermal brine. Neither method is entirely satisfactory and both are expensive.
This invention relates in particular to methods for controlling the deposition of silica solids from superheated hypersaline brine such as those encountered in the Salton Sea Known Geothermal Resource Area of the Imperial Valley in Calif. These geothermal brines, upon being brought to the surface for the purpose of extracting their energy, are depressurized and cooled and when this occurs the silica contained therein begins to precipitate from the brine. The residual brine becomes substantially more concentrated in dissolved chemicals, silica in particular, and the latter begins to precipitate and deposit. Silica precipitation continues until the concentration in the cooled, flashing brine reaches an equilibrium value.
Thus, supersaturated geothermal brines in wells found in the vicinity of Niland, Calif. may contain concentrations of silica in excess of 500 mg/l prior to flashing as compared to an equilibrium concentration of silica at 200.degree. F. and atmospheric pressure of about 180 mg/l The silica which precipitates out and deposits in the pipes and associated production equipment is amorphous. Severe fouling of the well, the pipes and the related equipment as well as scaling and corrosion is the inevitable result.
It is typical practice in operation of geothermal wells to dispose of the spent brines by injecting them into subterranean formations known as injection reservoirs through separate injection wells. Solid silica precipitating out from the geothermal brine tends to plug these injection wells and reservoirs and can in a short period of time interfere with brine disposal operations.